SCID-Xl is catastrophic immunodeficiency disorder caused by mutations within the common gamma chain (?c) gene. While stem cell transplantation using a matched sibling donor can be curative, most patients lack optimal donors leading to poorer outcomes. Gene replacement has many theoretical advantages as an alternative therapeutic approach for SCID-Xl; and pioneering clinical studies using gammaretroviral ?c delivery lead to both significant benefit as well as unanticipated adverse events due to viral enhancer triggered leukemogenesis. The overarching hypothesis of this PPG is that both the efficacy and safety of ?c gene delivery can be significantly improved using recombinant foamy virus (FV) based vectors. Studies in Project 1 are designed to test the hypotheses that yc FV vectors devoid of viral enhancers (with or without additional enhancer blocking elements flanking the transcriptional cassette) will exhibit levels of transgene expression sufficient for functional rescue in vivo while concurrently showing reduced genotoxicity. The aims of Project 1 are designed to test these hypotheses via detailed phenotypic, functional, and molecular analysis in both: 1) a small animal model of SCID-Xl and 2) hematopoietic stem cells (HSC) derived from SCID-Xl patients. Our specific studies will include efficacy and safety assessment of 1) EFIa-hu-?c FV vectors in vivo in myeloablated vs. non-myeloablated murine SCID-Xl recipients; and in alternative in vitro transactivation assays; 2) Preclinical and GMP-grade 1st generation ?c FV in transduced SCID-Xl patient CD34* BM cells; and 3) Candidate insulated 2nd generation ?c FV vectors in HSC from SCID-Xl mice and human patients. Project 1 will utilize all 4 Cores and will interface on multiple levels with work within both Projects 2 and 3. In conjunction with data derived from Projects 2 and 3, our studies will provide key data regarding efficacy, safety and optimal vector design for future 1st and 2nd generation SCID-Xl FV vector clinical trials.